Formalize some SourceFile parsing outputs

Currently when parsing a SourceFile, the parser
gets handed pointers so that it can write the
interface hash and collected tokens directly into
the file. It can also call `setSyntaxRoot` at
the end of parsing to set the syntax tree.

In preparation for the removal of
`performParseOnly`, this commit formalizes these
values as outputs of `ParseSourceFileRequest`,
ensuring that the file gets parsed when the
interface hash, collected tokens, or syntax tree
is queried.

tools/libSwiftSyntaxParser/libSwiftSyntaxParser.cpp

@@ -189,6 +189,12 @@ private:
     return getNodeHandler()(&node);
   }
 
+  Optional<SourceFileSyntax> realizeSyntaxRoot(OpaqueSyntaxNode root,
+                                               const SourceFile &SF) override {
+    // We don't support realizing syntax nodes from the C layout.
+    return None;
+  }
+
   void discardRecordedNode(OpaqueSyntaxNode node) override {
     // FIXME: This method should not be called at all.
   }